In recent years, electric devices, e.g. automotive electronics, electric tools, have been downsized, light-weighted, and operated at a higher efficiency. This market trend entails requirements for dc motors, which are supposed to be mounted to those devices as power sources, to be smaller in size, lighter in weight, and more efficient in operation.
A dc motor used in automotive electronics or household electric appliances produces greater electric sparks between a brush and a commutator slip (commutator segment) as the motor rotates at a greater rpm, becomes smaller in size and lighter in weight, and produces a greater output power. On top of that, the dc motor is required to have a longer service life as well as higher reliability.
This dc motor, in general, is formed of a stator forming a field magnet, and a rotor confronting the stator with an annular space therebetween. The rotor employs an armature equipped with a commutator and formed by winding a coil on a core. To drive this dc motor, it is needed to feed the armature with electric power, so that the brush connected to an outer power supply with lead wires should be in sliding contact with the commutator.
To increase the number of magnetic poles of magnets employed in the dc motor, it is necessary for various structural elements to be combined optimally. In this context, the structural elements include the number of magnetic poles of the coil relative to the number of magnetic poles of the magnets, a wire connection structure of the winding and so on.
In the wire connection structure for optimizing the motor performance which was built to meet the above needs, a ratio of useless coils to multiple armature coils tends to increase, so that the number of armature coils should be increased to obtain the necessary performance. A solution to the problem thus has unfortunately invited another problem. As a result, a greater change in the inductance has occurred, which adversely causes a voltage waveform and therefore sometimes shortens the service life of the motor. It is thus inevitable to employ a wire-wound structure although this structure somewhat degrades the motor performance. Noises caused by the solution to the same problem also become another problem.
The conventional dc motor discussed above is described hereinafter with reference to FIG. 8, which is an exploded view illustrating a winding method of the armature coil of the conventional dc motor. This conventional dc motor comprises the following structural elements:
a stator having four (4) field magnets;
a core having five (5) teeth;
a commutator having ten (10) segments;
an armature having coils wound on the teeth via wire-connections to the segments; and
a pair of brushes each of which is disposed orthogonally to each other and in sliding contact with the commutator.
The arc lengths of the brushes in sliding contact with the commutator are not greater than 5% of the circumference length of the commutator, namely, the arc length ≦π×A/20, where A is the outer diameter of the commutator.
FIG. 8 shows that brush B101 has a positive voltage and makes sliding contact with segment S103, and brush B102 has a negative voltage and makes sliding contact with segment S105. It will be discussed later how each of the coils are powered. Meanwhile, circled numbers 1, 2, 3 and 4 on both sides in FIG. 8 indicate that the same numbers are connected together.
A first electric current path starts from brush B101, and runs through S103, runs around tooth T105, runs through S108, runs around tooth T104, runs through S109, runs around tooth T103, runs through S104, runs around tooth T102, runs through S105, and arrives at brush B102.
A second electric current path starts from brush B101, and runs through S103, runs around tooth T101, runs through S102, runs around tooth T102, runs through S107, runs around tooth T103, runs through S106, runs around tooth T104, runs through S101, runs around tooth T105, runs through S110, runs around T101, runs through S105, and arrives at brush B102.
The dc motor has been improved to meet the required specification, and optimized structures of the dc motor have been proposed. Various techniques have been disclosed, e.g. patent literatures 1 and 2.
In a dc motor equipped with distributed windings, the winding passes over each tooth sequentially, so that coil ends, which is not involved in generating torque, become greater in size. The copper loss at the coil ends lowers the efficiency of the motor, and also causes to enlarge the size of the motor in the axial direction. As x signs in FIG. 8 show, an inverse current runs through some coils, and lowers the motor efficiency. The conventional dc motor discussed above thus still has a room for improvements in terms of achieving a smaller size, a lighter weight, and higher efficiency to satisfy the presently required specifications (e.g. Patent Literature 1).
A dc motor equipped with a concentrated winding or a wave winding also tries to optimize the combination of the number of magnet poles, the number of armature windings, and the number of commutator slips for pursuing higher torque, and achieve higher efficiency, and a compact size. However, as discussed above, this type of conventional dc motor also has a room for improvements in achieving a smaller size, a lighter weight, and higher efficiency to satisfy the presently required specifications (e.g. Patent Literature 2).    Patent Literature 1: Unexamined Japanese Patent Application Publication No. 2002-209362    Patent Literature 2: Unexamined Japanese Patent Application Publication No. S55-125069